HS1 constitutively associates with HPK1. Binding of HPK1 and HS1 in FD-EPO cells (A) or in SKT6 cells (B) before (lane 1) and after (lane 2) Epo stimulation or osmotic shock (lane 3). Anti-HS1 immunoprecipitates were probed with anti-HPK1 antibody (upper panels) or anti-HS1 antibody (lower panels). Arrows indicate HS1.